In the pollution control field, several diverse approaches have been used to remove sulfur oxides (SOx) and other contaminants from gas produced by the burning of a fossil fuel in order to comply with Federal and State emissions requirements. One conventional approach involves locating and utilizing fossil fuels lower in sulfur content and/or other contaminants. Another conventional approach involves removing or reducing the sulfur content and/or other contaminants in the fuel, before combustion, via mechanical and/or chemical processes. A major disadvantage to this approach is the limited cost effectiveness of the mechanical and/or chemical processing required to achieve the mandated reduction levels of sulfur oxides and/or other contaminants.
The most prevalent conventional approaches for removing sulfur oxides and/or other contaminants from gas streams involve post-combustion clean up of the gases. Several conventional methods have been developed to remove the sulfur dioxide (SO2) species from gases.
One conventional approach for removing SO2 from gas streams involves either mixing dry alkali material with the fuel prior to combustion, or injection of pulverized alkali material directly into the hot combustion gases to remove sulfur oxides and other contaminants via absorption or absorption followed by oxidation. Major disadvantages of this approach include fouling of heat transfer surfaces (which then requires more frequent soot blowing of these heat transfer surfaces), low to moderate removal efficiencies, poor reagent utilization, and increased particulate loading in the combustion gases which may require additional conditioning of the gas, such as humidification or sulfur trioxide injection, if an electrostatic precipitator is used for downstream particulate collection.
Another conventional approach for removing SO2 from gas streams, collectively referred to as wet chemical absorption processes and also known as wet scrubbing, involves “washing” the hot gases with an aqueous alkaline solution or slurry in a gas-liquid contact device to remove sulfur oxides and other contaminants. Major disadvantages associated with these wet scrubbing processes include the loss of liquid both to the atmosphere due to, for example, saturation of the gas and mist carry-over, and to the sludge produced in the process, and the economics associated with the construction materials for the absorber module itself and all related auxiliary downstream equipment (i.e., primary/secondary dewatering and waste water treatment subsystems).
Yet another conventional approach for removing SO2 from gas streams, collectively referred to as spray drying chemical absorption processes and also known as dry scrubbing, involves spraying an aqueous alkaline solution or slurry, which has been finely atomized via mechanical, dual-fluid or rotary type atomizers, into the hot gases to remove sulfur oxides and other contaminants. Major disadvantages associated with these dry scrubbing processes include moderate to high gas-side pressure drop across the spray dryer gas inlet distribution device and limitations on the spray down temperature (i.e., the approach to gas saturation temperature) required to maintain controlled operations.
There are several conventional methods for controlling emissions of nitrogen oxides (NOx), which include nitric oxide (NO), nitrogen dioxide (NO2), and dimers as principle components. Selective catalytic reduction (SCR) is the most common conventional approach. In this process, ammonia is injected and mixed with the gas at low to medium temperatures. The mixture then flows across a catalyst, often vanadium based over a stainless steel substrate, and the NOx is reduced to elemental nitrogen (N2). Deficiencies of conventional SCR systems include the high initial cost, the high cost of ammonia which is thermally or chemically decomposed, and the introduction of ammonia into the gas stream causing problems with the formation of ammonium bisulfate and ammonia slip to the atmosphere.
Selective non-catalytic reduction (SNCR) methods are also employed for controlling NOx emissions. In these processes, ammonia or urea is injected into hot gases resulting in a direct reaction forming N2. The problems with SNCR systems are the challenges with mixing and maintaining proper residence time and operating conditions for the reactions to take place optimally, sensitivity to changes in operating load, the high cost of ammonia which is thermally or chemically decomposed (even more than SCR's), and the introduction of ammonia into the gas stream causing problems with the formation of ammonium bisulfate and ammonia slip (as high as 50 ppm or higher) to the atmosphere. Dry injection of sodium bicarbonate (NaHCO3) may also remove NOx.
Wet chemical NOx reduction may use oxidants, such as hydrogen peroxide (H2O2). Hydrogen peroxide is an oxidizing agent for organic and inorganic chemical processing as well as semiconductor applications, bleach for textiles and pulp, and a treatment for municipal and industrial waste. Hydrogen peroxide is an effective chemical means of scrubbing nitrogen oxides and has been used for many years. The combined use of H2O2 and nitric acid (HNO3) to scrub both NO and NO2 is an attractive option because the combination handles widely varying rates of NO to NO2, adds no contaminants to the scrubbing solution or blow-down/waste stream, and allows a commercial product to be recovered from the process, such as nitric acid or ammonium nitrate.
Gas scrubbing is another common form of NOx treatment, with sodium hydroxide being the conventional scrubbing medium. However, the absorbed NOx is converted to nitrite and nitrate salts that may present wastewater disposal problems. Scrubbing solutions containing hydrogen peroxide are also effective at removing NOx, and can afford benefits not available with sodium hydroxide (NaOH). For example, H2O2 adds no contaminants to the scrubbing solution and so allows commercial products, such as nitric acid, to be recovered from the process. In its simplest application, H2O2 and nitric acid are used to scrub both NO and NO2 from many utility and industrial sources. In addition to the methods cited above in which NOx is oxidized to nitric acid or nitrate salts, other conventional approaches reduce NOx to nitrogen using hydrogen peroxide and ammonia.
Several other processes use hydrogen peroxide to remove NOx. The Kanto Denka process employs a scrubbing solution containing 0.2% hydrogen peroxide and 10% nitric acid while the Nikon process uses a 10% sodium hydroxide solution containing 3.5% hydrogen peroxide. Yet another process, the Ozawa process, scrubs NOx by spraying a hydrogen peroxide solution into the exhaust gas stream. The liquid is then separated from the gas stream and the nitric acid formed is neutralized with potassium hydroxide. Excess potassium nitrate is crystallized out and the solution reused after recharging with hydrogen.
H2O2 is used for the measurement of NO in the Standard Reference Method 7 of the Code of Federal Regulations (CFR) promulgated test methods published in the Federal Register as final rules by the United States Environmental Protection Agency (EPA). In this procedure, an H2O2 solution is used in a flask to effectively capture the NOx.
There are at least two primary reasons that H2O2 has not gained widespread use as a reagent for removal of NOx in utility and large industrial applications. One reason is that H2O2 is not a selective oxidant. Most of these sources also contain other species, primarily SO2, which are also effectively removed with hydrogen peroxide. Thus, a large quantity of H2O2 would be required compared to the amount of NOx removal sought. Even after a limestone scrubber, the amount of SO2 present in gas may be equal to or greater than the amount of NOx. Another reason that H2O2 has not gained widespread use is the cost, especially when much more H2O2 is required due to reactions with SO2, for example, which may be better done prior to the H2O2 stage.
The overall reactions are:3H2O2+2NO→2HNO3+2H2O  1)H2O2+2NO2→2HNO3  2)H2O2+SO2→H2SO4  3)
Chlorine oxide (ClO2) supplied at a rate of approximately 1.2 kg ClO2/kg NO is effective for rapidly converting over 90% of gas phase NO in the gas stream to NO2. This, of course, requires proper mixing conditions. ClO2 is a significantly stronger oxidizer than hydrogen peroxide, sodium chlorate, or sodium chlorite. Ozone is also a possible oxidizer, but has greater capital costs relative to ClO2 generators.
Sulfur dioxide reacts with chlorine dioxide in the gas phase to form sulfuric and hydrochloric acid.2ClO2+5SO2+6H2O→5H2SO4+2HCl  4)
Assuming SO2 is the dominant species in the ClO2 reaction in the presence of SO2 and NO, excessive amounts of ClO2 will be required to compensate for consumption by SO2. This will reduce the economic feasibility of using ClO2 for removing NOx.
None of these conventional approaches for scrubbing gas streams, like gas streams, simultaneously removes mercury, mercury compounds, and NOx, especially elemental mercury (Hg°) removal. Mercury is volatilized and converted to Hg° in the high temperature regions of fossil fuel combustion devices. As the gas cools, Hg0 is oxidized to Hg+2. In coal-fired combustors, Hg° may be oxidized to vapor phase mercuric oxide (HgO), mercuric sulfate (HgSO4), mercuric chloride (HgCl2), or some other vapor phase mercury compound.
Mercury may be captured, to a limited extent, using powdered activated carbon (PAC) sorbent. The activated carbon sorbent is injected into the gas stream, binds with the mercury in the gas, and captured downstream by a particulate matter control device. However, the mercury concentration in the gas stream may exceed the absorption ability of activated carbon sorbents. In addition, the performance of activated carbon sorbents may be adversely affected by low levels of chlorine in the gas. Carbon injection equipment is also relatively expensive.
Oxidized mercury (Hg+2 such as in the form of HgCl2), which are water-soluble, may be effectively captured in wet scrubbers used for SO2 control that use an alkali reagent. However, this process also requires supplemental additives, such as sodium hydrogen sulfide (NaHS) or other sulfides, to chemically bind with the mercury and form compounds like mercury sulfide (HgS). However, Hg° is insoluble in water and must be adsorbed onto a sorbent or converted to a soluble form of mercury that can be collected by wet scrubbing.
For these and other reasons, it is desirable to provide methods for removing nitrogen oxides, sulfur dioxide, and mercury-containing substances, such as mercury and mercury compounds, from gas streams that overcome the various problems associated with conventional methods for scrubbing gas streams.